[SPARK-9703] [SQL] Refactor EnsureRequirements to avoid certain unnecessary shuffles

This pull request refactors the `EnsureRequirements` planning rule in order to avoid the addition of certain unnecessary shuffles.

As an example of how unnecessary shuffles can occur, consider SortMergeJoin, which requires clustered distribution and sorted ordering of its children's input rows. Say that both of SMJ's children produce unsorted output but are both SinglePartition. In this case, we will need to inject sort operators but should not need to inject Exchanges. Unfortunately, it looks like the EnsureRequirements unnecessarily repartitions using a hash partitioning.

This patch solves this problem by refactoring `EnsureRequirements` to properly implement the `compatibleWith` checks that were broken in earlier implementations. See the significant inline comments for a better description of how this works. The majority of this PR is new comments and test cases, with few actual changes to the code.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7988 from JoshRosen/exchange-fixes and squashes the following commits:

38006e7 [Josh Rosen] Rewrite EnsureRequirements _yet again_ to make things even simpler
0983f75 [Josh Rosen] More guarantees vs. compatibleWith cleanup; delete BroadcastPartitioning.
8784bd9 [Josh Rosen] Giant comment explaining compatibleWith vs. guarantees
1307c50 [Josh Rosen] Update conditions for requiring child compatibility.
18cddeb [Josh Rosen] Rename DummyPlan to DummySparkPlan.
2c7e126 [Josh Rosen] Merge remote-tracking branch 'origin/master' into exchange-fixes
fee65c4 [Josh Rosen] Further refinement to comments / reasoning
642b0bb [Josh Rosen] Further expand comment / reasoning
06aba0c [Josh Rosen] Add more comments
8dbc845 [Josh Rosen] Add even more tests.
4f08278 [Josh Rosen] Fix the test by adding the compatibility check to EnsureRequirements
a1c12b9 [Josh Rosen] Add failing test to demonstrate allCompatible bug
0725a34 [Josh Rosen] Small assertion cleanup.
5172ac5 [Josh Rosen] Add test for requiresChildrenToProduceSameNumberOfPartitions.
2e0f33a [Josh Rosen] Write a more generic test for EnsureRequirements.
752b8de [Josh Rosen] style fix
c628daf [Josh Rosen] Revert accidental ExchangeSuite change.
c9fb231 [Josh Rosen] Rewrite exchange to fix better handle this case.
adcc742 [Josh Rosen] Move test to PlannerSuite.
0675956 [Josh Rosen] Preserving ordering and partitioning in row format converters also does not help.
cc5669c [Josh Rosen] Adding outputPartitioning to Repartition does not fix the test.
2dfc648 [Josh Rosen] Add failing test illustrating bad exchange planning.